A number of technical solutions are already known in which the capillary column is subjected to direct heating by means of an electrically conductive element which is set in contact with the capillary column and electrically powered in a controlled way.
Such an approach makes it possible to obtain various advantages, among which the considerable reduction in the electrical energy required for heating the capillary column and the rapid response of the system to the temperature programs that are set in the course of the analysis.
A number of the various possibilities of obtaining direct heating of the column are, for example, illustrated in U.S. Pat. No. 5,808,178 and in the corresponding international patent application No. WO 97/14957 in the name of Thermedics. Among the various solutions proposed, an assembly is illustrated comprising a column made of fused silica inserted in a tube made of steel, the latter having an internal diameter greater than the external diameter of the column for enabling insertion of the column itself. The steel tube is in turn coated with an insulating sheath made of woven glass fibres.
Another example is described in the U.S. Pat. No. 5,611,846 by Overton et al. For heating the column, this document suggests inserting the column into a sheath together with a conductive filament, or else inserting the column directly into a tube made of conductive material. In a publication by the same authors (“Novel Column Heater for Fast Capillary Gas Chromatography”; Overton et al.—Journal of Chromatographic Science—Vol. 34—December, 1996) it is emphasised that the proposed solution of inserting a capillary column directly into a tube made of conductive material is theoretically preferable for obtaining optimal heating of the column itself, but it is also noted that this solution has proven impracticable due to breakages that occur in the proximity of the sealed ends of the column assembly. This is mainly due to the different thermal coefficients of expansion of the materials.
Other examples of column assemblies of a direct-heating type may be found in the U.S. Pat. No. 5,114,439 by Yost et al., as well as in the U.S. Pat. No. 5,005,399 by Holtzclaw et al.. The columns illustrated in these documents are made of silica and coated with a layer of conductive material, in particular aluminium.
However, on account of the different coefficients of thermal expansion of the materials, there occur frequent breakages of the capillary column or of the conductive coating deposited thereon.
Another example of a direct-heating column for chromatography is illustrated in the U.S. Pat. No. 4,484,061 by Zelinka and Sims. Wound in a spiral on the column is at least one thin film of conductive material enclosed between two electrically insulating films. Fixing to the column during fabrication is ensured by an adhesive, and the assembly is then further coated with a spiral sheath. A construction of this kind, in addition to being particularly complicated and laborious, may prove far from suitable for withstanding high temperatures on account of the use of adhesives. In addition, in the absence of direct contact between the conductive heating material and the column, it is difficult to guarantee uniform heating of the column itself.
Given the above, the object of the present invention is to provide a chromatography-column assembly that enables perfectly uniform direct heating of the column throughout its length.
Another object of the present invention is to provide a chromatography-column assembly capable of withstanding of direct heating of the capillary column at high temperatures.
A further object of the present invention is to provide a particularly simple and economical method for making a direct-heating column assembly of the type referred to above.